The present invention relates to data transmission in digital systems. More specifically, the present invention relates to a driver that compensates for skin effect losses of the interconnection media by using a lower impedance when data switches at the maximum switching rate and using a higher impedance when data switches at less than the maximum switching rate.
In the art of digital signal processing, switching frequencies continue to increase. As is known in the art, the problems associated with transmitting high-frequency signals tend to be more difficult to solve when designing interconnect fabrics, which link together integrated circuits, circuit boards, and the like.
At high frequencies, such as 100 MHz and above, current is primarily carried by the outer skin of the conductor. Skin-effect resistance causes the attenuation of a conventional transmission line to increase with frequency. However, this attenuation is only present for the high-frequency components of the signal, and does not effect the low-frequency components. This phenomenon causes intersymbol interference, which degrades noise margin and reduces the maximum frequency at which the system can operate.
FIG. 1A shows a prior art transmission circuit 10, which includes a driver 11 having an output impedance of 20xcexa9. Driver 11 drives high signals toward VDD and drives low signals toward VSS. In FIG. 1A, typical values of 1.8 V for VDD and 0.0 V for VSS are shown. The driver is coupled to a transmission line 12. The other end of transmission line 12 is coupled to a receiver circuit, which is not shown in FIG. 1A, and is terminated with a 100xcexa9 resistor coupled to VDD and a 100xcexa9 resistor coupled to VSS.
FIG. 1B shows the Thevinen equivalent circuit 13 of the prior art transmission circuit 10 of FIG. 1A. The two terminal resistors in FIG. 1A can be modeled as a single resistor having a Thevinen resistance of 50106  and coupled to a Thevinen voltage of 0.9 V. Consider that driver 11 is driving transmission line 12 high for an extended period of time. The 20xcexa9 output impedance of driver 11 forms a voltage divider with the 50xcexa9 Thevinen resistence of the termination resistors. Accordingly, the receiver circuit will be provided with a DC signal of 1.54 V. Similarly, if driver 11 is driving transmission line 12 line low for an extended period of time, the voltage divider will provide the receiver circuit with a DC signal of 0.26 V.
If transmission line 12 where lossless, the signal provided to the receiver circuit would swing between the high and low DC values. Accordingly, the signal swing would be 1.28 V. However, because of skin effect losses of transmisssion line 12, the signal swing will be attenuated when the signal is switching at high frequencies. A nominal attenuation for a circuit such as that shown in FIG. 1A is 40%. Of course, the magnitude of attenuation will vary with frequency and the characteristics of transmission line 12. Applying the nominal attenuation to the lossless signal swing results in a signal swing of 0.77 V.
FIG. 2 shows a timing diagram of a signal 15 applied to driver 11 in FIG. 1A and the resulting waveform 16 observed at the receiver circuit. Assume that signal 15 has been low of an extended period of time, and therefore signal 16 has discharged down to the low DC value of 0.26 V, as described above. When the first pulse of a series of high-frequency pulses is transmitted at driver 11, signal 16 will rise 0.77 V, which is the signal swing calculated above. Accordingly, the pulse will rise to 1.04 V, which is just above the receiver detection threshold of 0.9 V. Note that the xe2x80x9ceye openingxe2x80x9d of this first pulse in signal 16 is very small, so there is little chance that the pulse will be properly detected by the receiver circuit.
As the series of high-frequency pulses continues to be transmitted, signal 16 centers itself about the average signal value, producing progressively better eye openings for the remaining pulses in the series. After the last pulse in the series, signal 15 remains low for several cycles, and signal 16 once again discharges down to the low DC value of 0.26 V.
After several low cycles, signal 15 once again goes high at pulse 17. Once again, the result is a pulse in signal 16 with a very small eye opening. Signal 15 then goes low for a cycle, and then goes high and remains high for several cycles. The result is that signal 16 charges up to the high DC value of 1.54 V.
At pulse 18, signal 15 goes low for a single cycle. Since the signal swing is 0.77 V, and the signal starts at 1.54 V, signal 16 will only fall to 0.77 V, which is just below the receiver threshold of 0.9 V. Accordingly, the pulse in signal 16 produced by pulse 18 in signal 15 also has a very small eye opening.
Now consider what happens if the circuit designer attempts to compensate for the small eye opening by using a lower output impedance in driver 11. The high-frequency signal swing of signal 16 will increase. However, the resulting voltage divider created by the termination resistors and the output impedance of the driver will cause the low DC value to become lower and the high DC value to become higher. Accordingly, while the signal swing is larger, the signal starts from a point farther from the receiver threshold, thereby negating the larger signal swing. Similarly, if the designer uses a higher output impedance in driver 11, the starting points will move closer to the receiver threshold, but the signal swing will decrease.
This problem of skin effect losses at high frequencies was addressed by William J. Dally and John Poulton in a paper entitled xe2x80x9cTransmitter Equalization for 4 Gb/s Signalingxe2x80x9d, which was first presented at the Proceedings of Hot Interconnects at Stanford University in August of 1996. This paper is hereby incorporated by reference. To solve the problem, Dally et al. propose using a 4 GHz finite impulse response (FIR) filter in a current-mode transmitter. The FIR filter increases the width and height of the xe2x80x9ceye openingxe2x80x9d, thereby making it easier to detect the pulse. In essence, the FIR filter prevents the transmission line from discharging down to a low level or charging to a high level. Unfortunately, the FIR filter consumes a relatively large amount of logic on an integrated circuit, and increases propagation delay.
The present invention is a driver circuit that compensates for skin effect losses in a transmission line by using a lower output impedance when data switches at the maximum switching rate, and using a higher output impedance when data switches at less than the maximum switching rate. As is known in the art, skin-effect resistance causes the impedance of a transmission line to be higher for high-frequency signal components. The present invention compensates for this effect by lowering the output impedance of the driver when transmitting high-frequency components having alternating data values, and using a higher output impedance when transmitting low frequency components having consecutive data values. When transmitting low-frequency consecutive high or low data values using a higher output impedance, the resulting voltage divider formed by the output impedance of the driver at the beginning of the transmission line and the termination resistors at the end of the transmission line causes the high and low DC levels at the end of the transmission line to move closer to the detection threshold of the receiver circuit, thereby causing the next isolated low or high pulse, respectively, to start from a point closer to the threshold. Furthermore, when transmitting high-frequency alternating values at the maximum switching rate using a lower output impedance, the resulting voltage divider produces a larger signal swing in the signal received by the receiver circuit. The result is that all pulses cross the receiver threshold with an excellent xe2x80x9ceye openingxe2x80x9d, thereby ensuring detection by the receiver circuit.
In one embodiment, the present invention uses a higher output impedance if the value being transmitted is the same as the previous value transmitted, and uses a lower output impedance if the value being transmitted is different than the previous value being transmitted. One pair of transistors are used to drive the output signal for every bit transmitted. A second pair of transistors are only used when the previous transmitted value is different from the value currently being transmitted. In this situation, the second pair of transistors operate in parallel with the first pair to lower the output impedance of the driver. If the value being transmitted is the same as the previous value transmitted, the second pair of transistors remain off and the data value is transmitted using a higher output impedance. The previous values are stored in latches, and a series of gates are used to compare the current and previous values to determine the impedance mode.
The present invention offers several advantages over the (FIR) finite impulse response filter technique taught in the prior art. First, the implementation is very simple and consumes relatively few gates of the integrated circuit. Also, the driver of the present invention introduces very little propagation delay. In addition, the present invention is easily extended to future and past generations of MOS technology.